1. Field of the Invention
The present invention relates to a bidirectional communication optical waveguide, which constitutes a transmitting and receiving module for bidirectionally transmitting a light signal using an optical fiber and a manufacturing method thereof.
2. Description of the Related Art
Conventionally, as a module using one optical fiber to bidirectionally transmit and receive light signals, a technology is generally used in which up and down signals having different wavelengths are utilized and the up and down signals are separated from each other by inserting a reflection plate having frequency selectivity within a module. However, in this configuration, it is necessary that the reflection plate having the frequency selectivity be separately provided in addition to a focusing lens, which results in inevitable cost increase in the module. Further, a transmitting and receiving module is also required, and the transmitting and receiving module is equipped with a combination of a dedicated light receiving device and a dedicated light emitting device according to the wavelengths used in the up and down signals, which results in complication and cost increase.
In order to avoid such problems, it is thought that the same wavelength is used for the bidirectional signals. For such techniques, for example, there is proposed a configuration in which the light receiving device is placed on a concentric circle of a surface light emitting device to perform bidirectional communication (for example, see Japanese Patent Application Laid-Open (JP-A) No. 58-191543). However, in this technique, transmission light from the light emitting device cannot be prevented from being reflected by an end surface of the optical fiber to return to the adjacent light receiving device on the same module side. Therefore, there arise many problems in receiving sensitivity.
There is also disclosed a technology in which a waveguide is used to realize bidirectional communication utilizing the same wavelength (for example, see JP-A No. 11-271548). The problem that the transmitted light from the light emitting device turns to a light receiving device on the same module is not generated in this technology. However, an input light beam from the optical fiber is inputted as stray light to the light emitting device side to some extent. In this technology, although the sub-waveguide is formed thinner than the main waveguide in order to prevent the stray light input, some slight stray light input cannot be avoided. When such stray light exists, output becomes unstable when using a semiconductor laser as a light emitting device, which generates a problem that a light signal level is changed. This problem is generated not only in an edge emitting type laser but also in a VCSEL (surface emitting laser).
There is also disclosed a technology in which an isolation function is realized by coupling single-mode waveguides having different refractive indexes and core diameters (for example, see JP-A No. 04-293004). However, because core shape conditions differ from each other, the isolation function is realized only by directly coupling the main waveguide and sub-waveguide, where core diameters differ from each other and refractive indexes differ from each other. Therefore, there is the problem that constraints on production being increased.
There is also disclosed a technology in which a pseudo-isolation function is realized by separately arranging a curved multi-mode waveguide having a tapered shape (see Journal of the Institute of Electronics, Information and Communication Engineers C-1, Vol. J82-C-1, No. 6, pp. 349-358 (1999)). All the refractive indexes of the waveguide cores are equal to one another, and the shapes of the waveguide cores are not optimized, so that there is the problem that coupling loss becomes large.
Thus, the module which causes the input and output signals from and to the optical signal to propagate stably without loss is required in order to realize the simply configured transmission and reception module for performing the bidirectional communication with the light signals having the same wavelengths. However, the sufficient technology for forming the module is not obtained yet.